Stereoisomers have the same structural formula but different arrangements of the atoms in space. Alkenes exhibit geometric isomerism. E/Z isomerism occurs in compounds with a C=C double bond if there are two different groups on each carbon atom of the C=C double bond. This is because there is restricted rotation of the carbon atoms about the double bond.The pi bond's electron density above and below the plane of the sigma bond makes the molecule rigid about the double bond. If the two high priority (higher atomic number at the first point of difference) groups are on the same side, the molecule is the Z-isomer. If they are on different sides (diagonally opposite) then the molecule is the E-isomer. This is known as cis (Z) and trans (E) isomers if both the high priority groups are the same.